1. Technical Field
Methods for enhancing the metal removal rate during chemical-mechanical polishing (CMP) of a semiconductor wafer are described herein. More particularly, the removal rate of a metal barrier layer on a semiconductor wafer is enhanced by employing a chelating agent in the chemical-mechanical polishing slurry during chemical-mechanical polishing of the semiconductor wafer.
2. Background of Related Art
Generally, semiconductor wafers include a plurality of circuits which form an integrated circuit. At some point when fabricating the integrated circuit on the semiconductor wafer, an oxide layer is formed on the wafer. Thereafter, the oxide layer is processed to pattern trenches or openings therein. Next, a metal barrier layer such as, for example, Ti/TiW, Ti/TiN or TaSiN is formed on the oxide layer by such techniques as physical vapor deposition (PVD) or chemical vapor deposition (CVD). Finally, a conductive layer, e.g., Al, W or Cu, will be deposited within the trenches or openings and over the top surface of the barrier layer. The semiconductor wafer is then polished to level its surface. During polishing, portions of the metal barrier layer and the conductive layer are removed from the top surface of the wafer.
One known polishing process is chemical-mechanical polishing (CMP) in which the semiconductor wafer is polished by employing a chemical-mechanical polishing apparatus. As seen in FIG. 1, a chemical mechanical polishing apparatus will ordinarily include a wafer carrier 15 for holding the semiconductor wafer 10. The wafer carrier 15 can be rotated during the polishing process by motor 17. CMP polishing platen 30, which carries the polishing pad 35, can be rotated by motor 37. The polishing slurry used during the process can be applied to polishing pad 35 via conduit 40.
Generally, the chemical-mechanical polishing process will involve holding the semiconductor wafer 10 against the rotating, wetted polishing surface of polishing pad 35. The polishing slurry is used to wet the polishing surface. The slurry may include a basic or acidic solution used as a chemical etch component in combination with an abrasive, such as alumina or silica particles. A rotating polishing head or the wafer carrier 15 is typically utilized to hold the wafer 10 against the rotating polishing platen 30 under controlled pressure. A backing film is optionally positioned between the wafer carrier 15 and the wafer. The polishing platen 30 is typically covered with a relatively soft wetted pad material such as blown polyurethane.
One drawback of the CMP process is that the different materials present at the surface of the wafer may polish at different rates. These different rates of removal can result simply from the different hardnesses of the materials or from different chemical interactions between the slurry and the materials. Thus, for example, the conductive layer may be easily removed by the abrasive action and an acidic slurry, while the metal barrier layer is not subject to such removal. This unwanted, excessive isotropic removal of the conductive metal layer can leave large metal areas "dished" toward the center. Thus, the goal of achieving a flat surface comprised of metal and insulator at various locations across the wafer surface is not achieved.
U.S. Pat. No. 5,676,587 discloses a two step CMP process. The first step employs a standard alumina-based CMP slurry to remove the metal barrier layer and the conductive layer but stops before it reaches the oxide layer. The second step employs a neutral pH silica and water or silica-based CMP solution to remove the remainder of the metal barrier layer.
It would be desirable to provide an easily implemented CMP method that substantially equalizes the removal rate of the metal barrier layer and the conductive layer from a semiconductor wafer during a chemical-mechanical polishing process to provide a flat surface comprised of metal and insulator regions across the surface of the wafer or alternately it would be desirable to provide a method of enhancing the removal rate of the metal barrier layer in the aforementioned two step CMP process.